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post #1 of 32 Old 10-10-2013 Thread Starter
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Stability question

I am currently boat shopping and one of my chief concerns is stability in heavy weather. Are there any other stability calculations other than Angle of Vanishing Stability and Capsize Screen Value that I could take into consideration when judging a hull form for stability?

Thanks in advance!
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post #2 of 32 Old 10-10-2013
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Re: Stability question

Forget about Capsize Screen Value unless you are buying an old design and even so....

If you are talking about modern boats, you can ask the boat stability curves (or try to find them) but besides having to had the knowledge to read them they will tell you only a part of the history. They tell you about static stability that is important but don't say nothing about dynamic stability that is as important.

Here you have some thoughts about the issue:

Interesting Sailboats

Besides stability considerations there are other important factors regarding heavy weather sailing like the possibility of reefing the boat from the cockpit, the systems you use (regarding jamming possibility), the sails you have and can deploy from the cockpit (cutter rig) and boat motion.

It remains to be known what you call heavy weather. Many of the things that make a boat good for heavy weather sailing makes the boat less good in normal sailing conditions that are probably where you are going to use it 99% of the time. Boats are a compromise and unless you will want to sail it in extreme latitudes the right compromise for most sailors is not a boat specially set for heavy weather.

All boats in the market today of 36ft and over if well sailed and prepared with the right equipment can take some heavy weather without problems even if they are not designed taking that as main design directive. This does not mean that they are all the same: some are better than others on that situation.


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Last edited by PCP; 10-10-2013 at 11:16 AM.
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post #3 of 32 Old 10-10-2013
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Re: Stability question

Is this your first boat? What size are you considering? Are you planning on extended cruising most of the time? That is, are you expecting to frequently be underway while the heavy weather blows through?

If you're like many of us who mostly daysail with the occasional weekend or annual week's trip, I don't know that I'd focus TOO much on heavy weather. Yes, you want the boat to be able to survive a storm, but most boats will do that just fine, especially if you aren't underway.

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post #4 of 32 Old 10-10-2013
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Re: Stability question

Stole this from smurphny, but great presentation of stability.

www.usna.edu/Users/naome/phmiller/offshore.ppt

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post #5 of 32 Old 10-10-2013
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Re: Stability question

Those numbers are fine and valid as references but there is a lot more to it than those static numbers. So long as you choose a "normal" boat with reasonable shape and avoid any of the extreme designs you should be fine. I think your safety in heavy weather depends a lot more on you than it does those numbers. Learn how and when to reduce sail. Would you pass on a boat that has LOPS (limit of positive stability) of 123 degrees compared to another boat with a LOPS of 126 degrees? I wouldn't. In extreme conditions I'm not sure that a LOPS of 120 degs makes any difference compared to a LOPS of 128 degs. Stick with boats that have a beam to length ratio (L/B) between 3.3 and 4.00. Avoid boats with an L/B near 3.00 or below. Pick a boat with a D/L around 220 to 270. Recognize that shoal draft will almost certainly raise your VCG and reduce righting moment (Rm)

It is very difficult to generalize about stability. I think there is a preoccupation with static stability numbers today. Probably because computers make it relatively easy to get those numbers. But they are Dependant for their accuracy on an accurate VCG and this has to come from a painstaking weight study and there is no computer short cut for that.

Years ago I worked for a very well known design office for a year. In that year we did one stability study and that was only to determine the Rm for rating purposes.

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Re: Stability question

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Originally Posted by jimgo View Post
Is this your first boat? What size are you considering? Are you planning on extended cruising most of the time? That is, are you expecting to frequently be underway while the heavy weather blows through?

If you're like many of us who mostly daysail with the occasional weekend or annual week's trip, I don't know that I'd focus TOO much on heavy weather. Yes, you want the boat to be able to survive a storm, but most boats will do that just fine, especially if you aren't underway.
Looking for a cruising vessel to potentially circumnavigate; will start off on East coast of North America and the Carribean. Not my first boat and I am just looking for some baseline hullform numbers on vessels which would otherwise meet my specs. I am two-3 years from pulling the trigger on this so I'm just in the number crunching stage

Thanks for the prompt reply!
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Re: Stability question

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Originally Posted by RainDog View Post
Stole this from smurphny, but great presentation of stability.

www.usna.edu/Users/naome/phmiller/offshore.ppt
Thanks rain dog!

I'll study this when I get a chance.
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post #8 of 32 Old 10-10-2013
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Re: Stability question

Bob's guideline numbers make a lot of sense.. and those numbers are easily arrived at. Further a large range of typically available production boats fall into those brackets.

The Catalina 30 and 36 are among the least tender boats readily available and are fine coastal cruisers - Island hopping would be fine too, but many will say they are not truly trans-ocean designs. Great place to start, though, but not the only good candidates.

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post #9 of 32 Old 10-10-2013
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Re: Stability question

I'm not in the position of choosing a circumnavigating boat at this point, however, I would suggest you attack this from the other side. Instead of starting with numbers that you would use to screen in or screen out certain boats, look for experts opinions on boats that would make good circumnavigators, then see if they fit your idea of a good offshore boat. In the end, you will probably be limited somewhat by budget and geography anyway.

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post #10 of 32 Old 10-11-2013
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Re: Stability question

Ted Brewer's "Comfort Ratio" seems to be a useful indicator of yacht performance in our experience. For more information see (click on) Crunching Numbers.

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